This invention relates to a process and apparatus having a uniform magnetic field and more particularly to apparatus having a uniform magnetic field within which experimentation or sample analysis can be conducted. By the term "uniform" as used herein with reference to the static magnetic field is meant a field having a constant and known gradient and when the gradient is zero the field is "homogeneous".
A variety of apparatus utilize a static magnetic field within which experimentation or sample analysis is conducted Examples of such apparatus include high resolution nuclear magnetic resonance apparatus, magnetic imaging apparatus, ion cyclotron mass spectrometry apparatus or the like. In this apparatus, it is desirable to render uniform the static magnetic field in order to minimize anomalies in measurements.
Nuclear magnetic resonance (NMR) is the resonance effect of an alternating magnetic field at right angles to a stronger static magnetic field to change the orientation of nuclear magnetic moments within the static magnetic filed. The alternating magnetic filed generally is applied as the magnetic component of a radio frequency field which can be applied in a continuous manner ever a frequency range or which can be applied as pulses at a fixed frequency. The applied magnetic field causes an induced signal from the sample which signal is uniquely characteristic for a given nucleus. In order for the measured induced nuclear magnetism to accurately reflect the characteristics of the nuclear sample, it is essential that the static magnetic field be uniform over the sample volume.
The use of currents in coils of various geometry to establish magnetic filed uniformity (homogeneity) is an established paratic. Such practice has resulted in design and utilization of space efficient coils for simultaneous control of various axial gradients of the magnetic field in which each coil addresses more than one axial gradient of the field. Conventional practice of design and utilization of coils for control of radial gradients of the magnetic field have extended to gradients of relatively lower order and typically uses designs in which each coil assembly is used for only one radial gradient of the magnetic field; a spatially inefficient approach if a relatively large number of gradients are to be controlled.
Prior to the present invention, those anomalies in the homogeneity of the static field of nuclear magnetic resonance apparatus not treatable with conventional coils were averaged out by spinning the sample within the most homogeneous volume of the static field. This procedure is undesirable since it requires mechanical apparatus which may itself produce anomalies during measurements due to variations in spin velocity and to slight movements of the spin axis. In addition, the use of apparatus for spinning the sample is undesirably cumbersome.
It has been proposed in U.S. Pat. No. 4,654,592 to analyze a plurality of samples by NMR wherein the samples are arrayed axially along z axis of a room temperature bore located within a superconducting magnet. Such apparatus also requires the loading of discrete samples within the sample cell in advance; and inconvenience. The apparatus permits analysis of a plurality of samples simultaneously. However, the apparatus requires that the samples be spun continuously during analysis thereby necessitating the use of suitable mechanical spinning apparatus.
It would be desirable to provide a method and apparatus for analyzing a plurality of samples by NMR simultaneously or concomitantly in a high resolution NMR apparatus avoids the necessity of spinning the sample. Furthermore, it would be desirable to provide such an NMR apparatus having the uniform static magnetic field over a relatively large fraction of the available room temperature bore so that a plurality of samples can be analyzed simultaneously or concomitantly. In addition, it would be desirable to provide a uniform static magnetic field in apparatus requiring a static magnetic field in order that larger samples can be analyzed.